Did you feel that?
If you're standing, or sitting, somewhere in Michigan, the ground beneath your feet is rising — or falling, depending on where you are — all the time. Little by little. Almost imperceptibly, but steadily.
The surface of the Great Lakes region is still recovering from the aftereffects of the last Ice Age, when woolly mammoths and mastodons grazed on grasslands here along with white-tailed deer. The ground is bouncing back from the lifting of the staggering weight of the glaciers, miles-thick ice that covered the region from about 85,000 to 11,000 years ago, that helped carve the Great Lakes through their scouring of rock and dirt as they moved.
"The enormous weight associated with that ice literally pushed the lithosphere, the Earth's upper crust, down into the upper mantle — the very dense, viscous matter, the lava and magma that come out in volcanic eruptions," said Michigan State University geography professor Alan Ford Arbogast.
"Because the mantle was viscous and could give, it was pushed away. Now, with the glaciers gone, the mantle is in the process of returning, and the landscape is rebounding."
The changes may only be a small fraction of an inch per year, but over time — and over vast Great Lakes surfaces — it matters, for the precision that freighters need to navigate to how much hydroelectric power can be produced, to where development can occur along shorelines.
This bounce-back phenomenon, called isostatic rebound, isn't much in metro Detroit, but it's magnified the farther north you go in Michigan. It all depends on the amount of ice that weighed down the land, and how relatively recently the glaciers receded from the area. Around Sault Ste. Marie, the surface is rising about 1 foot every century. But that's nothing compared to Hudson Bay, in northern Ontario, about 600 miles due north from the Soo.
The Hudson Bay region had some of the heaviest glacial ice during the last Ice Age and was the last area to have the ice melt away. The land surface there is rising more than a half-inch per year — more than 4 feet per century.
For surveyors relying on pinpoint precision, isostatic rebound is a complication. Teams of researchers in both the U.S. and Canada are presently in the years-long process of doing a grand reset of Great Lakes levels to account for the changes — the third such reset since 1955, required every 25 to 35 years. Known as the International Great Lakes Datum, it's relied upon to help Great Lakes ships safely and effectively navigate the lakes; it's used in laws and rules pertaining to shoreline development and habitat preservation, and it helps the U.S. and Canada govern water flows and usage for things such as hydroelectric power generation, codified in their binational agreements.
The difference may only be an inch or so of water, said Daniel Roman, chief geodist for the National Geodetic Survey, a division of the National Oceanic and Atmospheric Administration that provides the framework for all GPS and other positioning activities in the U.S.
"But across the entire surface of Lake Superior, an inch of water is a lot of water," he said. "A lot more water is a lot more power to hydroelectric companies."
Lake Superior had a foot of change in its water levels from the first International Great Lakes Datum in 1955 to its 1985 update, said Laura Rear McLaughlin, mapping and charting program manager for NOAA's Center for Operational Oceanographic Products and Services.
"We're expecting to see another foot of change this time around," she said.
Michigan has particularly interesting dynamics from isostatic rebound, as the area near what became the state's southern border is also about the farthest extent of the last glacial period, more than 20,000 years ago, before the ice began to recede. The sheer weight of the ice, which averaged more than a mile thick, pushed down the Earth's crust and displaced the mantle below, pushing it into portions of what became southernmost Michigan, northern Ohio and Indiana, making the ground there rise in elevation. Thus, as northern Michigan rises from the weight of the glaciers being removed, areas along the southern end of the Great Lakes region are seeing surface elevations drop — about 3.5 inches per century in areas near Gary, Ind., and south of Cleveland.
The International Great Lakes Datum update includes seven years of collecting lake depth data at points all throughout the Great Lakes.
"There's a lot of seasonal variability on the Great Lakes," Roman said. "You want an average over a significant period of time."
The 1955 datum sampled levels at 193 different locations; budget constraints and access issues took the sample locations down to 53 sites for the 1985 datum, McLaughlin said.
U.S. and Canadian crews are slated to install 21 seasonal lake-level monitoring gauges along the Great Lakes this year and next year, she said.
"We're trying to get back to the 140 sites that we visited in 1955, but not in 1985," McLaughlin said. "We need to find partners to get to that 140."
The 1955 and 1985 efforts measured lake levels against an agreed-upon reference zero-point: Pointe au-Père, near Rimouski, Quebec, on the St. Lawrence River downstream from the Great Lakes. The newest update, however, will use a new mean sea level reference point known as a geoid. It takes into account the irregular shape of water on Earth because of slight variations in gravity's pull between mountains and valleys, and at the equator versus nearer the poles because of the effects of the Earth's rotation. Using complex math and gravity readings, the model measures surface elevations to an even higher degree of accuracy.
"GPS technology will make this update much more accurate and efficient," Roman said. "In the past, you just sent a level crew up the St. Lawrence River with surveyors tools, and told them to march 100 meters and measure vertically over and over. There was quite an expense associated with that."
The International Great Lakes Datum is essential to setting the charted lake depths relied upon in shipping navigation, said Glenn Nekvasil, vice president of the Lake Carriers Association, a trade group representing U.S. freighters operating on the Great Lakes.
"We need to know how deep the water is, obviously," he said. "You go too deep and you can put a hole in your ship."
It also matters economically, in how precisely a ship can maximize its cargo load, Nekvasil said. "These ships gain or lose anywhere from 50 to 270 tons of cargo for each inch of draft," the vertical distance from the waterline to the bottom of a ship's hull, he said. Ensuring the ship leaves port as full as it can be while safely avoiding the lake bottom requires very accurate, up-to-date depth charts, he said.
The International Great Lakes Datum is also used in law. Michigan's Great Lakes Submerged Lands Act refers to the datum to help define such lands and the state's regulation of them. Illinois' Rivers, Lakes and Streams Act requires permits for shoreline construction, utilizing the datum. Indiana uses it to define the ordinary high watermark in its shoreline regulations.
Work on the updated International Great Lakes Datum will continue until its expected publication in 2025.
"It's amazing to me," Arbogast said. "It's been 10,000 years since the glacial ice has been in the U.P., and the land is still rebounding from that."
Contact Keith Matheny: 313-222-5021 or email@example.com. Follow on Twitter @keithmatheny.